The dystrophin-glycoprotein complex (DGC) is a multi-subunit protein complex expressed at the sarcolemma of skeletal, cardiac, and smooth muscle fibers (reviewed in Campbell, Cell 80: 675-679 (1995), and Straub and Campbell, Curr. Opin. Neurol. 10: 169-175 (1997)). The DGC is currently known to be composed of at least nine proteins including dystrophin, the syntrophins, .alpha.- and .beta.-dystroglycan, .alpha.-, .beta.-, .gamma.-, and .delta.-sarcoglycans and sarcospan. One of the functions of the DGC is likely to provide a structural link between the extracellular matrix and the actin cytoskeleton through interactions of dystrophin with filamentous actin, and .alpha.-dystroglycan with the extracellular matrix component laminin, thereby maintaining the stability of the sarcolemma under contractile forces (Ervasti and Campbell, J. Cell Biol. 122(4): 809-823 (1993); and Campbell, Cell 80: 675-679 (1995). Recent evidence suggests that the DGC may play other roles in normal muscle physiology through interactions with cell signaling molecules or other proteins at the sarcolemma.
Sarcospan is the most recently cloned component of the DGC (Crosbie et al., J. Biol. Chem. 272: 31221-31224 (1997). Hydropathy plots predict that the protein has four transmembrane domains with an extracellular loop extending between transmembrane domains 3 and 4 (Scott et al., Genomics 20: 227-230 (1994); Crosbie et al., J. Biol. Chem. 272: 31221-31224 (1997). Dendogram analysis designates sarcospan as a member of the tetraspan superfamily, also known as the transmembrane-4 superfamily or the tetraspanins (Heighway et al., Genomics 35: 227-230 (1996); Wright and Tomlinson, Immunol. Today 15: 588-594 (1994). Tetraspan proteins are thought to function as molecular facilitators, mediating interaction between proteins at the plasma membrane. The tetraspans have also been implicated in cell adhesion, migration, and proliferation (Wright et al., Immunol. Today 15: 588-594 (1994); Maecher et al., FASEB 11: 428-442 (1997). Sarcospan is tightly associated with the sarcoglycans to form a subcomplex of the DGC (Crosbie et al., J. Cell Biol. 145: 153-165 (1999). The function of the sarcoglycan-sarcospan complex is currently unknown. One hypothesis is that it stabilizes .alpha.-dystroglycan at the membrane. Another hypothesis is that the sarcoglycan-sarcospan complex may be important in the signaling functions of the DGC, a possibility which remains relatively unexplored.
Defects in components of the DGC have been implicated in muscle disorders manifested by muscle weakness and wasting. Currently, it is known that six forms of muscular dystrophies are caused by primary genetic defects within components of the DGC. These include Duchenne and Becker muscular dystrophies, the most prevalent forms of muscular dystrophies that are caused by mutations in the dystrophin gene and four forms of autosomal recessive limb-girdle muscular dystrophies (LGMD2-C, -D, -E and -F) caused by primary mutations in each of the four sarcoglycan genes (Straub and Campbell, Curr. Opin. Neurol. 10: 169-175 (1997). Additionally, mutations in the laminin-.alpha.2 chain cause a severe form of congenital muscular dystrophy. Recent data suggests that dystroglycan is important in basement membrane formation (Henry and Campbell, Cell 95: 859-870 (1998) and dystroglycan-null mice die at a very early embryonic stage (Williamson et al., Hum. Mol. Genet. 6: 831-841 (1997). It is likely that human-null mutations in the dystroglycans would also lead to an early embryonic lethality. In contrast, disruption of the .alpha.1-syntrophin gene in mice was not lethal, and also did not result in muscle degeneration (Kameya et al., J. Biol. Chem. 274: 2193-2200 (1999). However, neuronal nitric oxide synthase, which is usually localized at the sarcolemma through .alpha.1-syntrophin, was not found at the sarcolemma in these animals (Kameya et al., J. Biol. Chem. 274: 2193-2200 (1999).
To investigate the function of sarcospan, a sarcospan-deficient mouse was generated and characterized. The mouse generated was observed to exhibit an obese phenotype. Obesity in humans is a widespread and serious disorder, affecting a high percentage of the adult population in developed countries. Few persons suffering from this disorder are able to permanently achieve significant weight loss. This failure to treat obesity may be at least partially attributed to the complexity of the disease. An understanding of the genetic factors that underlie obesity may aid in treatment. Animal models are useful in developing this understanding. Current mouse models for obesity include obese (ob), agouti (wt), tubby (tub), fat and diabetes (db). These animal models are extremely useful for their ability to simplify the heritability of an otherwise very complex trait.